Testing the stator of an electric motor

To test the stator of an electric motor, the following steps should be carried out in sequence:

1. 01Gather data on the fault and the operating regime.
2. 02Safely disconnect and isolate the motor.
3. 03Carry out an external inspection.
4. 04Inspect the terminal box and the connections.
5. 05Check the winding for continuity.
6. 06Measure the phase resistance.
7. 07Measure the insulation resistance to the frame.
8. 08If needed, determine the absorption ratio or the polarization index.
9. 09Check the insulation between phases.
10. 10Measure the inductance or the impedance of the phases.
11. 11Carry out a surge test for interturn defects.
12. 12Perform a high-voltage test, if this is required by the test programme.
13. 13Check the stator core.
14. 14Inspect the slot wedges, the end windings and the bandages.
15. 15Compare the electrical parameters of all phases.
16. 16If possible, run a no-load test on the motor.
17. 17Draw a conclusion on whether the motor can continue in service or needs repair.

The final scope of testing depends on the voltage, power and design of the motor, its condition, and the reasons it was taken out of service.

The stator of an electric motor is not just a winding.

Its main elements include:

• the frame;
• the active steel (core) stack;
• the slots;
• the winding;
• the slot insulation;
• the phase-to-phase insulation;
• the end windings;
• the slot wedges;
• the bandages;
• spacers and support elements;
• the leads;
• the terminal box;
• temperature sensors;
• heaters;
• the ventilation system;
• in some machines — water or air coolers.

A defect in any of these elements can damage the winding.

A full or partial stator check is carried out:

• during scheduled maintenance;
• before putting the motor into service;
• after prolonged storage;
• after moisture ingress;
• after flooding;
• after severe overheating;
• after the overcurrent protection has tripped;
• after a short circuit in the network;
• when phase currents are uneven;
• when heating is increased;
• when there is a smell of burnt insulation;
• when insulation resistance has decreased;
• when there is a strong electromagnetic hum;
• after a winding repair;
• after a rotor has been replaced or repaired;
• after the rotor has been in contact with the stator;
• after a failure of the driven mechanism;
• before buying a used motor;
• after prolonged overload;
• when an interturn short circuit is suspected.

Diagnostics should allow the following to be found or ruled out:

• a phase open circuit;
• an open circuit in a parallel branch;
• poor contact at a connection;
• an interturn short circuit;
• an inter-coil short circuit;
• a phase-to-phase short circuit;
• a breakdown of the winding to the frame;
• reduced insulation resistance;
• moisture ingress;
• contamination;
• thermal ageing;
• mechanical damage to the insulation;
• an incorrect phase connection;
• an unequal number of turns;
• an incorrect winding pitch;
• damage to the leads;
• loosened slot wedges;
• movement of the end windings;
• failure of the bandages;
• damage to the active steel;
• shorted core laminations;
• local overheating of the core;
• a loosened core stack;
• contaminated ventilation ducts;
• a fault in the temperature sensors.

Depending on the scope of diagnostics, the following are used:

• a multimeter;
• a milliohmmeter or microhmmeter;
• a megohmmeter;
• an absorption-ratio meter;
• a polarization-index instrument;
• an LCR meter;
• an inductance meter;
• a winding surge tester;
• a high-voltage test set;
• an instrument for checking the active steel;
• a thermal imager;
• a clamp meter;
• a power-quality analyser;
• a vibration analyser;
• a tachometer;
• an endoscope;
• a dial indicator;
• an air-gap gauge;
• a technical stethoscope;
• no-load test equipment.

Not every check can be safely carried out on site at the plant. Surge and high-voltage tests must be performed by trained personnel using suitable equipment.

1. Gather the nameplate data

Before testing begins, the following must be recorded:

• the motor type;
• the serial number;
• the power;
• the rated voltage;
• the rated current;
• the frequency;
• the speed;
• the connection diagram;
• the insulation class;
• the operating regime;
• the ingress protection rating;
• the cooling method;
• the year of manufacture;
• the repair history;
• the results of previous measurements.

Comparison with previous results is often more valuable than the assessment of a single measurement.

2. Establish the reason for the check

It is necessary to establish:

• what happened before the shutdown;
• whether the protection tripped;
• what the phase currents were;
• whether there was overheating;
• whether a smell appeared;
• whether there was smoke;
• whether the motor ran on two phases;
• whether the driven mechanism jammed;
• whether the start was prolonged;
• whether the connection diagram was changed;
• whether the terminal box was repaired;
• whether moisture got in;
• whether the motor was running from a variable frequency drive.

Without a failure history, even correct measurements can be misinterpreted.

3. Safely disconnect the motor

Before starting work, it is necessary to:

• disconnect the power supply;
• lock out reclosing;
• confirm the absence of voltage;
• discharge any capacitive circuits;
• disconnect the power cables;
• disconnect the variable frequency drive;
• disconnect the capacitors;
• disconnect sensors and auxiliary circuits if they can affect the measurement;
• earth the leads after high-voltage testing;
• identify and label all leads.

It is best to start the check before cleaning and disassembly, since traces of dust, moisture, oil and overheating can point to the source of the fault.

The following must be inspected:

• the frame;
• the feet and flanges;
• the ventilation openings;
• the terminal box;
• the cable entries;
• the seals;
• the earthing;
• traces of grease leakage;
• traces of water;
• corrosion;
• cracks;
• traces of overheating;
• a change in varnish colour;
• the smell of the insulation.

Particular attention is paid to:

• black or brown dust;
• green traces of copper corrosion;
• white deposits left by moisture;
• arc traces;
• melted spots;
• cracks in the insulators;
• loose bolts;
• damaged cable lugs.

Defects that arise in the terminal box are often mistaken for stator winding damage.

It is necessary to check:

• the condition of the terminal board;
• the tightness of the nuts;
• the links;
• the cable lugs;
• traces of heating;
• oxidation;
• cracks in the insulators;
• the clearances between phases;
• compliance with the connection diagram;
• the labelling of the leads;
• the condition of the internal lead-out cables.

Poor contact can cause:

• local heating;
• a voltage drop;
• a current unbalance;
• charring of the insulation;
• a phase open circuit;
• a phase-to-phase short circuit.

Before electrical measurements it is advisable to disconnect the links and separate the phases, if the design allows this.

The simplest check is done with a multimeter in resistance or continuity mode.

There must be electrical continuity between the leads of each phase.

A lack of continuity can indicate:

• a phase open circuit;
• a break in an internal connection;
• damage to a lead;
• poor contact at a terminal;
• the built-in thermal protection tripping, if it is included in the circuit;
• an open circuit in a parallel branch.

However, an ordinary multimeter does not always detect a partial open circuit in a parallel branch, because the overall electrical continuity of the phase is preserved.

Why measure the resistance

Comparing the phase resistance allows the following to be detected:

• an open circuit;
• a partial open circuit in a parallel branch;
• poor contact;
• an incorrect connection;
• an unequal number of turns;
• a soldering or welding defect;
• winding asymmetry.

For low-resistance windings an ordinary multimeter is often not accurate enough. It is advisable to use a milliohmmeter or a microhmmeter with a four-wire measurement scheme.

How to take the measurement

It is necessary to:

1. 01Disconnect the power cables.
2. 02Remove the links.
3. 03Clean the contact surfaces.
4. 04Measure each phase with the same method.
5. 05Record the winding temperature.
6. 06If needed, correct the results to a common temperature.
7. 07Compare the values with each other and with previous data.

The resistance of a copper winding changes with temperature, so measurements of a cold and a hot winding cannot be compared without a correction.

What an inequality of resistances means

Possible cases:

• one phase has a higher resistance — poor contact, an open circuit in part of the parallel branches, a reduced conductor cross-section;
• one phase has a lower resistance — an incorrect number of turns, a connection error, partial shorting;
• the value is unstable — a loose contact or a damaged lead;
• all values differ significantly from the nameplate or previous data — a diagram error, temperature, or a repair that changed the parameters.

A small difference can be a design feature, especially with leads of different lengths. It must be assessed with reference to the specific motor’s diagram.

What a megohmmeter shows

A megohmmeter checks the ability of the insulation to resist the flow of direct current.

Typically measured:

• each phase to the frame;
• all phases connected together, to the frame;
• phase to phase, if the windings are separated;
• auxiliary circuits;
• heaters;
• sensors, if their testing is permitted by the manufacturer.

Important conditions

Before the measurement it is necessary to:

• disconnect the electronics;
• disconnect the variable frequency drive;
• disconnect sensitive sensors;
• clean the terminals;
• record the temperature;
• make sure the winding has no residual charge;
• discharge the winding after the measurement.

The test voltage is chosen according to the voltage class, the condition of the machine and the applicable instructions.

Why a single value is not enough

Insulation resistance depends on:

• temperature;
• humidity;
• contamination;
• the insulation area;
• the duration of the applied voltage;
• the motor design;
• the operating history.

The result should therefore be compared:

• with regulatory requirements;
• with previous measurements;
• with the other phases;
• with the results after drying or cleaning;
• with the absorption ratio and the polarization index.

When direct voltage is applied, the resistance of sound, dry insulation often increases with time.

The following can be used for the assessment:

• the ratio of resistances measured at different points in time;
• the polarization index;
• the absorption ratio.

These indicators help assess:

• moisture;
• contamination;
• the overall condition of the insulation;
• the degree of ageing.

However, for low-voltage motors with modern insulation the indicators are not always unambiguous. They should not be used without taking into account the design, the temperature and the manufacturer’s recommendations.

If the phases are brought out separately by design, the insulation resistance can be checked:

• U–V;
• V–W;
• W–U.

Low resistance between phases can indicate:

• contamination of the terminal box;
• moisture;
• damage to the phase-to-phase insulation;
• carbonisation;
• a defect in the end windings;
• a phase-to-phase short circuit.

If the winding’s neutral point is connected inside the motor and is not accessible, the individual phases cannot be fully separated without dismantling the connection.

An interturn short circuit occurs between adjacent turns of the same coil.

In that case:

• both turns have almost the same potential relative to the frame;
• the phase-to-frame insulation can remain sound;
• the megohmmeter shows a high resistance;
• the motor already has a local electrical defect.

A high insulation resistance reading therefore does not guarantee the absence of an interturn short circuit.

Detecting it requires other methods:

• a surge test;
• comparison of inductance;
• current analysis;
• thermal-imaging inspection;
• testing at reduced voltage;
• a special coil check.

Inductance depends on:

• the number of turns;
• the connection diagram;
• the condition of the magnetic circuit;
• the rotor position;
• the presence of shorted turns.

Comparing the phases can help detect:

• an interturn short circuit;
• an error in the number of turns;
• an incorrect connection;
• asymmetry of the magnetic circuit.

It is important to take the measurements:

• at the same rotor position;
• at the same test-signal frequency;
• with the same connection scheme;
• at the same temperature.

In an assembled asynchronous motor, the rotor position can affect the inductance, so it is worthwhile taking several measurements at different shaft positions.

What this method is

A surge tester applies short high-voltage pulses to the winding and compares the shapes of the resulting oscillatory waveforms.

The method makes it possible to detect:

• interturn defects;
• inter-coil defects;
• an unequal number of turns;
• an incorrect connection;
• weak points in the insulation;
• defects that do not yet show up in a direct-current measurement.

How the result is assessed

In a symmetrical three-phase winding, the waveforms of the phase signals should be close to each other.

Differences can show up as:

• a shift in the oscillation frequency;
• a change in amplitude;
• a change in waveform shape;
• an unstable breakdown;
• a sudden divergence of the signals as the voltage is raised.

The interpretation must take into account the winding diagram, the accessibility of the phases, the rotor position and any design asymmetry.

Can a surge test damage the winding

An incorrectly chosen voltage or a departure from the procedure can place a dangerous load on aged insulation.

It is therefore necessary to take into account:

• the voltage class;
• the repair history;
• the age of the winding;
• the condition of the insulation;
• the manufacturer’s recommendations;
• the approved test programme.

A high-voltage (hipot) test checks the dielectric strength of the insulation:

• phase to frame;
• phase to phase;
• auxiliary circuits.

The method can reveal:

• a weak point in the slot insulation;
• damage to the leads;
• a defect in the phase-to-phase insulation;
• contamination;
• a fault introduced during a repair.

But the test should not be applied automatically to every old motor. Its appropriateness and the voltage level are determined by:

• the condition of the machine;
• the type of repair;
• the rated voltage;
• the requirements of the standards;
• the test programme.

After the test, the winding must always be discharged and earthed.

Removing the rotor allows a much more complete inspection.

It is necessary to check:

• the internal surface of the stator;
• traces of rotor contact;
• the condition of the slot wedges;
• the position of the coils;
• the end windings;
• the bandages;
• the spacers;
• the impregnation;
• contamination;
• cracks;
• traces of partial discharge;
• corona damage;
• overheating;
• the condition of the leads;
• the ventilation ducts.

Darkening of the whole winding

This can indicate:

• prolonged overload;
• insufficient cooling;
• a high ambient temperature;
• frequent starts;
• an incorrect operating regime.

Darkening of a single phase

Possible causes:

• a phase open circuit during operation;
• voltage unbalance;
• an incorrect connection;
• a defect in a parallel branch;
• a local electrical fault.

Local burning of a coil

Possible causes:

• an interturn short circuit;
• mechanical damage;
• a foreign object;
• a defect in the slot insulation;
• a loosened wedge;
• a partial discharge.

Burning in a slot

This can mean:

• a breakdown to the core;
• damage to the slot insulation;
• contact of a conductor with the active steel;
• movement of a coil;
• a sharp edge;
• damage that occurred during winding installation.

Damage to the end windings

Possible causes:

• electrodynamic forces;
• vibration;
• loosened bandages;
• phase-to-phase contact;
• a mechanical impact;
• contamination;
• moisture.

White or green deposits

This can indicate:

• moisture;
• chemical contamination;
• copper corrosion;
• an aggressive environment.

Cracks in the varnish

Possible causes:

• ageing;
• overheating;
• vibration;
• an unsuitable impregnation system;
• cyclic thermal expansion.

The slot wedges hold the winding in the slots.

It is necessary to check:

• that all the wedges are present;
• how tightly they are seated;
• cracks;
• displacement;
• traces of rubbing;
• scorching;
• looseness.

A loose wedge can cause:

• movement of a coil;
• vibration;
• insulation damage;
• electromagnetic noise;
• an interturn short circuit;
• a breakdown to the core.

The assessment method depends on the design. Simple tapping can only serve as a preliminary check and does not replace a specialised inspection.

The end windings are exposed to:

• electrodynamic forces;
• vibration;
• thermal expansion;
• the effect of the airflow;
• contamination;
• the effect of moisture.

The following are checked:

• displacement of coils;
• loosened bandages;
• traces of rubbing;
• contact between phases;
• cracks;
• scorching;
• the condition of the spacers;
• the condition of the connections;
• the clearance to the frame;
• the condition of the sensors.

It is especially important to assess the points where the coils exit the slots, since significant mechanical stresses act there.

Why the core also needs to be checked

The stator stack is built up from thin insulated laminations of electrical steel.

If short circuits occur between the laminations, eddy currents and local losses increase.

Consequences:

• local overheating;
• destruction of the winding insulation;
• increased no-load current;
• reduced efficiency;
• a strong electromagnetic hum;
• repeated damage to a new winding.

Rewinding the stator without checking the core after severe burning can lead to a repeat failure.

When checking the core is mandatory

Particular attention is paid to the core:

• after a winding has burned out;
• after the rotor has been in contact with the stator;
• after the winding was removed thermally;
• after a fire;
• in the case of local overheating;
• in the case of damage to the teeth;
• after machining;
• in the case of increased no-load current after a repair.

Checking methods

The following can be used:

• visual inspection;
• a ring flux (loop) test;
• thermal-imaging inspection;
• specialised electromagnetic scanning;
• localised checking of suspect zones.

During the test they look for:

• local hot spots;
• uneven heating;
• damage to the teeth;
• a loosened core stack;
• shorted laminations;
• traces of melting.

A loose core stack can cause:

• vibration of the laminations;
• electromagnetic hum;
• local heating;
• destruction of the interlaminar insulation;
• mechanical damage to the winding.

Signs:

• metal dust;
• traces of lamination movement;
• buzzing;
• local vibration;
• loosened clamping elements;
• cracks in the fastenings.

The following are checked:

• the condition of the flexible leads;
• soldered joints;
• welded joints;
• crimped connections;
• the transition points from the winding to the cable;
• insulating sleeves;
• mechanical fastening;
• traces of heating;
• cracks.

A poor internal contact can show normal electrical continuity with no load, yet overheat at the operating current.

The stator may be fitted with:

• thermistors;
• thermocouples;
• resistance temperature detectors;
• thermal switches;
• PTC sensors.

It is necessary to check:

• the continuity of the circuit;
• the resistance;
• that the type is correct;
• the insulation from the winding and the frame;
• that the labelling is correct;
• the operation of the protective relay.

Without removing the rotor, it is usually possible to carry out:

• an inspection of the terminal box;
• a phase resistance measurement;
• an insulation resistance measurement;
• a phase-to-phase check;
• determination of the polarization index;
• an inductance measurement;
• a surge test;
• analysis of the operating currents;
• thermal-imaging inspection;
• motor current analysis;
• a no-load test;
• vibration and noise analysis.

However, without disassembly it is difficult to fully assess:

• the condition of the slot insulation;
• looseness of the wedges;
• damage to the end windings;
• traces of rotor contact;
• the condition of the internal surface of the core.

Diagnostics of a running machine can include:

• measuring the phase currents;
• measuring the phase voltages;
• determining the unbalance;
• analysing the active and reactive power;
• thermal-imaging inspection;
• measuring the temperature with built-in sensors;
• monitoring vibration;
• analysing noise;
• analysing the current spectrum;
• monitoring the speed;
• measuring the no-load current;
• assessing the acceleration time.

Suspicious signs are:

• significant unevenness of the phase currents;
• increased no-load current;
• local heating of the frame;
• a rise in temperature of one phase;
• an unstable electromagnetic hum;
• torque pulsations.

Diagnostics while running does not replace disconnection and electrical tests, but it helps determine the direction of the search.

Phase currents must be measured simultaneously or under the same conditions.

Uneven currents can be caused by:

• voltage unbalance;
• a phase open circuit;
• poor contact;
• an interturn short circuit;
• an incorrect connection;
• a rotor defect;
• an uneven load in special circuits;
• a fault in the drive.

Therefore, before concluding that the stator is faulty, the supply voltages must be checked.

After a repair, or during in-depth diagnostics, the motor is tested without mechanical load, if this is permitted by the design.

The following are monitored:

• the phase currents;
• the symmetry;
• the speed;
• the direction of rotation;
• the noise;
• the vibration;
• the temperature;
• the acceleration time;
• the no-load current.

An increased no-load current can indicate:

• an incorrect number of turns;
• an incorrect connection diagram;
• core saturation;
• a defect in the active steel;
• an uneven air gap;
• mechanical friction;
• an incorrect voltage.

A multimeter can only perform a basic check.

What can be checked:

• the presence of an open circuit;
• an approximate equality of resistances;
• the condition of some sensors;
• the continuity of the leads;
• a gross short circuit to the frame.

What cannot be reliably checked:

• the condition of high-voltage insulation;
• an interturn short circuit;
• the dielectric strength;
• the quality of the slot insulation;
• the condition of the core;
• a partial open circuit in a parallel branch in a low-resistance winding;
• a weak contact that only shows up at high current.

The conclusion "the multimeter shows the same value, so the stator is fine" is therefore incorrect.

The general procedure:

1. 01Disconnect the motor from all external circuits.
2. 02Confirm the absence of voltage.
3. 03Separate the phases, if possible.
4. 04Connect one lead of the instrument to the winding.
5. 05Connect the other to a cleaned point on the frame.
6. 06Apply the test voltage.
7. 07Record the reading and the temperature.
8. 08Repeat for the other phases.
9. 09Check between the phases.
10. 10Discharge the winding after the test.

The exact test voltage and the minimum acceptable result must be determined from the documentation and the test programme.

A combination of methods is used for a reliable check:

• comparison of the phase resistances;
• comparison of the inductances;
• a surge test;
• analysis of the phase currents;
• analysis of the current spectrum;
• thermal-imaging inspection;
• testing at reduced voltage;
• a localised check of the coils after disconnecting the circuit.

For many winding types, the most informative method is a surge comparison of the phases or of individual coil groups.

After a new winding has been made, it is advisable to carry out:

• a check of the diagram;
• a check of the number of turns;
• a check of the pitch;
• a check of the phasing;
• a phase resistance measurement;
• an inductance measurement;
• a check of the interturn insulation;
• a check of the insulation to the frame;
• a check of the phase-to-phase insulation;
• a high-voltage test;
• a check of the sensors;
• an inspection for jamming;
• a check of the bandages;
• a check of the end-winding geometry;
• a check after impregnation and drying;
• a no-load test;
• monitoring of the currents and heating.

It is especially important to compare the no-load current after the repair with the nameplate or archived values.

After severe damage it is not enough simply to remove the old winding and wind a new one.

It is necessary to:

1. 01Record the location of the burnout.
2. 02Establish the root cause.
3. 03Inspect the teeth and the back of the core.
4. 04Check the interlaminar insulation.
5. 05Carry out a test of the active steel.
6. 06Eliminate local short circuits.
7. 07Check the geometry of the slots.
8. 08Restore the damaged areas.
9. 09Only after this carry out the rewinding.

If the core has significant local losses, the new winding can overheat again in the same place.

Drying can be effective if the low insulation resistance is caused mainly by moisture, and the winding has no:

• cracks;
• carbonisation;
• a breakdown;
• heavy contamination;
• an interturn short circuit;
• mechanical damage.

After drying, the following must be repeated:

• the insulation resistance measurement;
• the absorption ratio;
• the polarization index;
• the inspection;
• a surge test, if needed.

An increase in resistance after drying does not rule out a local electrical defect.

Cleaning and re-impregnation can be considered if:

• the insulation is contaminated;
• the varnish coating is degraded;
• the winding is mechanically sound;
• there is no interturn defect;
• there is no severe overheating;
• the core is sound.

Impregnation cannot restore:

• charred insulation;
• damaged turns;
• an incorrect diagram;
• a breakdown between phases;
• a damaged core.

A full rewind is usually required in the case of:

• an interturn short circuit;
• a phase-to-phase breakdown;
• a breakdown to the core;
• significant thermal ageing;
• charring;
• numerous local defects;
• severe damage to the end windings;
• destruction of the slot insulation;
• the impossibility of a reliable local repair;
• an incorrect previous winding;
• significant contamination that cannot be removed without destroying the insulation.

The decision must take into account the power, the criticality of the mechanism, the availability of a spare motor and the cost of an emergency shutdown.

A local repair can be appropriate if:

• the defect is precisely localised;
• the rest of the winding is in satisfactory condition;
• the core is undamaged;
• access to the defect is possible;
• the repair will provide the necessary electrical and mechanical strength;
• process documentation exists;
• the machine does not have critically high criticality, or the repair is agreed with the customer.

A full set of electrical tests is required after a local repair.

The stator should not be allowed to run when there is:

• low insulation resistance;
• an unstable breakdown;
• divergence of the surge waveforms;
• significant asymmetry of the resistances;
• an open circuit in a parallel branch;
• loosened slot wedges;
• damaged bandages;
• traces of rotor contact;
• local overheating of the core;
• charred insulation;
• damaged leads;
• an incorrect diagram;
• a faulty thermal protection on a critical motor;
• an unestablished cause of a previous failure.

Do not rely on the megohmmeter alone for a conclusion

A high insulation resistance does not rule out an interturn short circuit.

Do not apply a megohmmeter to a variable frequency drive

This can damage the power electronics.

Do not test sensors with high voltage without checking their permissible parameters

Temperature sensors and electronic modules can be damaged.

Do not compare resistances at different temperatures

The temperature of the copper significantly affects the result.

Do not use a multimeter for an accurate comparison of low-resistance phases

The resistance of the probes and contacts can be comparable to the resistance of the winding.

Do not carry out a high-voltage test without a programme

An incorrectly chosen voltage can damage aged insulation.

Do not rewind the stator without checking the core after a burnout

Damaged active steel can destroy the new winding.

Do not clean the winding with aggressive substances

The solvent can damage the varnish, the bandages and the insulating materials.

Do not dry the motor with uncontrolled heating

Exceeding the permissible temperature accelerates ageing of the insulation.

Do not perform repeated trial starts of a faulty motor

Every start creates large thermal and electrodynamic loads.

1. 01Checking only the insulation resistance.
2. 02Not separating the phases.
3. 03Not recording the temperature.
4. 04Not checking the terminal box.
5. 05Using a multimeter instead of a milliohmmeter.
6. 06Not checking the inductance.
7. 07Not carrying out a surge test.
8. 08Not checking the active steel.
9. 09Ignoring the slot wedges.
10. 10Not inspecting the end windings.
11. 11Not checking the internal connections.
12. 12Not analysing the cause of previous overheating.
13. 13Not comparing the results with archived data.
14. 14Not checking the supply voltage.
15. 15Treating uneven currents as an unambiguous sign of a stator defect.
16. 16Not taking the condition of the rotor into account.
17. 17Not checking the temperature sensors.
18. 18Not testing after the machine has warmed up.

In practice, most mistakes arise from trying to get an answer using a single instrument.

The megohmmeter shows a high resistance, but the motor overheats

Possible causes:

• an interturn short circuit;
• an incorrect number of turns;
• a rotor defect;
• overload;
• voltage unbalance;
• insufficient cooling;
• damage to the active steel.

The phase resistances are equal, but the currents differ

It is necessary to check:

• the phase voltages;
• the inductance;
• the surge waveforms;
• the diagram;
• the rotor;
• the air gap;
• the starter contacts.

The insulation resistance recovered after drying

This is a positive result, but it is necessary to establish:

• where the moisture came from;
• whether the seals are damaged;
• whether the heaters work;
• whether there is any carbonisation;
• whether the interturn insulation is sound.

After rewinding, the motor has a high no-load current

Most often checked:

• the number of turns;
• the pitch;
• the diagram;
• the phasing;
• the connection of the parallel branches;
• the active steel;
• the air gap.

The winding burned out in one slot

The core in that zone must be checked with particular care. Simply fitting new slot insulation may not be enough.

Basic level

For an initial check:

1. 01Inspection.
2. 02Terminal box.
3. 03Continuity check.
4. 04Phase resistance.
5. 05Insulation resistance.
6. 06Phase-to-phase check.
7. 07Analysis of the operating currents.

Extended level

Additionally:

1. 01Polarization index.
2. 02Inductance.
3. 03Surge test.
4. 04Thermal-imaging inspection.
5. 05Vibration analysis.
6. 06No-load test.

Full factory-level defectation

Additionally:

1. 01Removal of the rotor.
2. 02Inspection of the slots and end windings.
3. 03Checking the wedges.
4. 04Checking the active steel.
5. 05High-voltage tests.
6. 06Checking the sensors.
7. 07Drawing up the report and the repair conclusion.

It is advisable to include in the report:

• the motor type and number;
• the nameplate data;
• the reason for the check;
• the date;
• the temperature;
• the condition before cleaning;
• the inspection results;
• the winding diagram;
• the phase resistances;
• the insulation resistance;
• the absorption ratio;
• the polarization index;
• the inductance;
• the surge test results;
• the high-voltage test results;
• the condition of the core;
• the condition of the wedges;
• the condition of the end windings;
• the condition of the sensors;
• the no-load test results;
• the phase currents;
• the temperatures;
• the defects found;
• the recommended repair;
• the final conclusion.

Photographs of the defects should preferably be attached to the report.

For critical motors it is advisable to keep a history of:

• the insulation resistance;
• the polarization index;
• the phase resistance;
• the currents;
• the voltages;
• the temperatures;
• the acceleration time;
• the number of starts;
• overloads;
• emergency shutdowns;
• the results of thermal-imaging inspections;
• repairs;
• drying;
• impregnation;
• sensor replacements;
• the parameters of the variable frequency drive.

A trend in the indicators often makes it possible to detect deterioration earlier than a single measurement would.

• How to test a rotor?
• How to check an electric motor winding?Coming soon
• How to find an interturn short circuit?
• How to measure insulation resistance?
• How to use a megohmmeter?
• What is the polarization index?
• How to check the stator active steel?Coming soon
• How to check the slot wedges?Coming soon
• How to check an electric motor after rewinding?
• Why does the stator overheat?
• Why do electric motor windings burn out?
• Why does an electric motor draw increased current?
• Why does a motor hum?
• How to carry out a no-load test on an electric motor?
• How to check an electric motor before installation?

Elektropromremont LLC carries out comprehensive diagnostics, repair and testing of the stators of industrial electrical machines.

The scope of work includes:

• external and internal defectation;
• measurement of the winding resistance;
• measurement of the insulation resistance;
• determination of the absorption ratio;
• determination of the polarization index;
• checking the phase-to-phase insulation;
• measurement of the inductance;
• surge testing of the windings;
• high-voltage testing;
• diagnosis of interturn defects;
• checking the slot wedges;
• inspection of the end windings;
• checking the active steel;
• thermal-imaging inspection;
• local winding repair;
• full rewinding;
• replacement of the slot and phase-to-phase insulation;
• impregnation and drying;
• lead repair;
• installation and checking of temperature sensors;
• no-load testing;
• load testing;
• preparation of reports and technical conclusions.

A complete stator check is a comprehensive procedure, not a single megohmmeter measurement.

To reliably assess the condition of the stator, the following must be checked:

• the winding for an open circuit;
• the phase resistance;
• the insulation to the frame;
• the insulation between phases;
• the interturn condition;
• the inductance;
• the correctness of the diagram;
• the condition of the leads;
• the slot wedges;
• the end windings;
• the sensors;
• the active steel;
• the operating currents;
• the heating;
• the motor’s behaviour on no load.

The basic principle of diagnostics is to compare several independent indicators.

For example:

• a high insulation resistance does not rule out an interturn defect;
• equal phase resistances do not guarantee an equal number of turns;
• uneven currents do not always mean a stator fault;
• a new winding will not run reliably if the core is damaged.

The purpose of the check is not only to establish whether the motor can run now, but also to determine whether its further operation will be safe and reliable.